WLANs (Wireless Local Area Networks) utilize RF (Radio Frequency) signals or light signals to connect mobile endpoints to each other or to a centralized gateway and transmit data over a wireless medium between the physical endpoints or between a mobile endpoint and an endpoint on a network that is connected to the WLAN. In 1997 the IEEE published standards for WLANs under the title of 802.11 (also known as “Wi-Fi”). The IEEE 802.11b protocol has gained popularity over the past few years and deployment of 802.11b networks is expected to increase significantly in the near future. Currently, most of these networks are used for data access from laptop computers and personal digital assistants (PDAs). The basic hardware setup of an IEEE 802.11 network is the Basic Service Set (BSS), which is merely a number of endpoint stations that communicate with one another. And ESS is larger than a BSS and can be a combination of BSSs or a BSS and other associated network nodes, components, and LAN lines. Using a WLAN to place voice phone calls using VoIP (Voice over Internet Protocols) over WLAN is also expected to grow significantly in the near future. However, VoIP over WLAN presents a unique set of problems that must be addressed prior implementing this technology.
There exists a plurality of 802.11 standards that each use different frequency bands and have varying data transmission speeds. The original IEEE 802.11 standard supported wireless interfaces operating at speeds of up to 2 megabyte per second (Mbps) in the 2.4 GHz radio band. By using different modulation techniques, IEEE 802.11b raised the data transmission rates to 11 Mbps, while 802.11a supports up to 54 Mbps transmission rates at a 5 GHz frequency. The IEEE 802.11 μg is developing a standard for data transmission rates of 54 Mbps at the 2.4 GHz frequency.
WLANs under 802.11 use media access control (MAC) protocols to transmit between wired and wireless devices. Each wireless network card is assigned a MAC address used to identify the station. In a BSS, IEEE 802.11 enables wireless mobile stations (STAs) to communicate through a wireless network interface directly with each other or with other stations through an access point. An access point (AP) is a centralized gateway providing message and power management and access to an external LAN (Local Area Network) and/or the Internet.
The access to wireless networks is controlled by coordination functions. The distributed coordination function (DCF) provides access similar to Ethernet CSMA/CA access. The DCF determines if the RF link between devices is clear prior to transmitting. Stations use a random backoff after every frame to avoid collisions. Endpoint stations provide MAC Service Data Units (MSDUs) after detecting no current transmissions. The MSDUs functions to transmit data frames to the proper endpoint station.
FIG. 1 illustrates a schematic diagram of an exemplary WLAN enterprise network under IEEE 802.11 protocols. One basic service set (BSS) has a wireless access point (AP1) 12 and a second BSS has a wireless access point (AP2) 14. An AP acts as a bridge for data with wireless STAs that are associated with that AP. An enterprise network typically has multiple BSSs and multiple APs distributed throughout an office complex or among floors on buildings so that a STA may be operated from nearly anywhere in a complex or building. Each AP in a BSS has an RF propagation broadcast area that has an effective range based upon broadcast power, natural signal attenuation, and interferences. AP1 12 has an RF propagation area defined exemplarily by coverage ring 13, and AP2 14 has an RF propagation area defined exemplarily by coverage ring 15. A WLAN may also be used to send voice data signals using a WIPP (Wireless Internet Protocol Phone or IP Phone) 16 that transmits data signals using voice protocols, such as voice over Internet Protocol (VoIP). Notebook computer 18 is associated with AP1 12 using a wireless network interface card and transmits data using IEEE 802.11 protocols. Both APs 12, 14 are connected to an internal corporate Intranet 20. The Internet 26 may be accessed through intranet 20 and gateway 22 or alternatively through AP1 12 through a Radius authentication server 24.
Referring to FIG. 2, a user carrying WIPP 16 that is associated with AP2 14 may want to walk into an area near AP1 using traverse path 40. If AP1 and AP2 are part of the same BSS or WLAN, a “handoff” occurs when WIPP 16′ enters AP1's coverage area 13, ends association with AP2, and associates with AP1. Thus, when the WIPP 16 enters area 13, the association of wireless data transfer from WIPP 16 moves from AP2 to AP1. At least two APs are always involved in a handoff. The AP that the STA is moving away from is called the prior-AP (here AP2) and the AP the STA moving towards is called the posterior-AP (here AP1). A handoff process permits a user to move a STA around a large WLAN containing multiple BSSs, and hence multiple APs, while continuing to seamlessly use the STA without dropping its connection to the WLAN.
A problem with handoffs, however, occurs with latency during the handoff process. The probing delay is the dominating component in handoff times. Studies, such as An Empirical Analysis of the IEEE 802.11 MAC Layer Handoff Process (Mishra, Shin, and Arbaugh, University of Maryland, College Park, Md.) have shown that over 90% of the delay time during a handoff is consumed by the probing process. During this latency period the STA cannot transmit or receive data. This is especially a concern for a WIPP 16 that sends and receives phone calls over the WLAN. Typical handoff latency is of the order of 100-300 msec (milliseconds). The probing process during a handoff causes a STA to probe all channels to search for a suitable AP with which to connect. An exemplary STA driver may scan all possible channels sequentially starting from zero. This delay becomes especially relevant for real-time applications like voice and video over WLAN.
Handoff delay can be explained by analyzing the default handoff procedure of a STA. Once a STA determines that it needs to do a handoff, it would typically scan all channels for finding an AP to connect to and then connect to the most “suitable” one among the available choices. There are two problems with this approach. First, scanning all channels every time a handoff is needed consumes a considerable amount of time. Second, the “suitable AP selection” is usually chosen due to the signal strength, i.e. if the SSIDs, security settings, etc. are similar, than an AP with the highest signal strength would be selected. Selecting an AP simply because it has the highest signal strength will not always yield expected results for seamless networking, since ESSs are usually designed with a bit of overlap to provide uninterrupted connectivity. This overlap means that AP1 may be better than AP2 in an overlapping region but outside this overlap, each one would dominate its own coverage area.